Mclachlan



Feb. 14, 1956 D, McLAcH-LAN, JR 2,735,018

X-RAY MI'CROSCOPE Filed Feb, 19. 1955 2 Sheets-Sheet l mam Num INVENTOR.DA/V M6 LACHLA/V Jr:

MWL W@ A TTORNEY.

Feb. 14, 1956 D. MoLAcHLAN, JR 2,735,018

X-RAY MICROSCOPE Filed Feb. 19. 1953 2 Sheets-Sheet 2 INVENToR 04NMdm-HMM, JA.

ATTORNEY United States Patent O X-RAY NIICROSCQPE Dan McLachlan, Jr.,Salt Lake City, Utah, assignor to American Cyanamid Company, New York,N. Y., a corporation of Maine Application February 19, 1,953, Serial No. 337,711

4 Claims. (Cl. 25053) ation it is possible to use optical systems inwhich either refracting lenses or reflecting mirrors are used for imageformation, because in this range of radiation there are availablematerials having widely different transmission rates (indices ofrefraction) for the energy in question and also materials which are soopaque to the radiation that satisfactory mirror surfaces can beprepared on which a large portion of the radiant energy can bereilected.

Streams of electrons can be converged and diverged by electrical meansto produce effects similar to refractive lenses in the visible lightrange. However, X-rays have not been hitherto readily available for useas a radiation for enlarged image formation. The reason for this failurelies in the fact that there are no sufficiently great differences inspeeds of propagation of X-rays in different substances so thatrefracting systems are not feasible. At the same time, the penetrationof X-rays is so great that the problem of an X-ray mirror has beeninsoluble except for a few special cases of substantially grazingincidence. X-rays being electromagnetic Waves, of course, cannot readilybe converged and diverged by magnetic or electrostatic lenses, as can bestreams of electrons.

This leaves only one property of materials which can be considered inthe design of X-ray image formers, namely, their ability to diffract.However, the use of crystals as diflracting elements is subject to aserious limitation which has made their use for image formationimpractical. Suitable difracting crystals can be bent, but they can bebent in only one direction, compound curvature setting up stresses whichbreak the crystals when they are bent in a different direction. Whenbent in one direction to form a cylindrical surface, it is possible touse large crystals to effect convergence and divergence of X-rays bydiffraction, but the rays are converged in only one dimension. In otherwords, the X-ray optical elements thus produced behave in a mannersimilar to cylindrical mirrors which permit forming enlarged images onlyof lines extending parallel to the axis of the cylinder and do notpermit the formation of enlarged images of many dierent kinds with anysatisfactory degree of definition.

rlfhe present invention utilizes the phenomenon of X- ray diffraction,but overcomes the difficulties encountered with the cylindricaldiffractors referred to above, permitting convergence in an additionaldimension which makes practical the formation of enlarged images ofuseful definition by X-ray diffraction.

The problem of making a diracting surface which s 2,735,018 PatentedFeb. 14, 1956 will be capable of converging X-raysA presents a seriousbut not insoluble problem. The fact that it is not possible to bendlarge crystals in two directions may be overcome by using materialswhich have a microcrystalline surface. The random orientation of thecrystals in such surfaces results in their behavior, under suitablecircumstances, as if they formed a diffraction mirror. In other words,convergence and divergence of X-rays can be effected by diffraction in amanner to give a result similar to that obtained by the reflection of alonger wave length radiation, Vsuch as light.

The manner in which X-rays may be used to effect magnification of thinsections may be understood by reference to Figures l and 2 whichillustrate different embodiments of my invention.

Fig. 1 shows the method of vmaking enlarged images by the simplest meanspertinent to this invention. Fig. la illustrates discs having a singlewire therebetween and Fig. 1b illustrates discs having a plurality ofwires'therebetween. Fig. 2 illustrates a mechanical device helpful indesigning the X-ray target. With reference to Fig. l, enclosure E is aspecial X-ray tube housing inside of which is an ordinary filament F forgenerating electrons by thermal emission. The voltage between thefilament and housing may be 5 kilovolts or as high as 40 kilovolts,depending upon the radiation desired and the materials used in thewindow and the discs (see below). The window T is unusual in that itplays a double role of both window and target. It is cooled by thepassage of cooling liquid'through the cooling jacket C.

The electrons generated` by the filament Fare brought against thetarget-window by the voltage difference existing between the lament andthe target-window. The stoppage of the electrons bythe window causes ageneration of X-rays, both continuous and characteristic, over theplanar area of the window.

The object O-O' to be enlarged is placed adjacent to the exteriorsurface of thewindow. The'X-rays which emerge'from the window will thenpass through the sample or object, and the variationy of, X-rayintensity over the bealmvarea is a function ofthe variation of opacityof the object to X-rays.

The disc D in Fig. 1 isa round flat plate, relatively opaque to X-ra-ys,surrounded by a second concentric disk.4 A circular wire is positionedin the annular opening between the twofdisks and held in place by smallwire supports to the outer concentric disk. Y

If the ray originating from `the point g, on the object., strikes thewire at d, af portion of the ray will continue in the direction of gcand a portion will be diffracted or bent in the direction h, wherel itstrikes the film or plate. The angle between the lines dg@ andA dh is`20and is characteristic of the material chosenfor the wire. The ray.originating from the point gf on the object and which strikes the pointd, must have a portion which continues in the direction gfc and thediifractedqPOrtion must be bent the same angle 29v as was the rayoriginating from g, but the bent ray from g strikes theflm at h'. Thusthe object gg is enlarged to hh on, the plate S. All points around thecircular WireV behave in` a manner identicalV to that described for thebehavior at the point d.

It is apparent from Fig. l that to obtain sharp focusing of the image ona flat screen, the surface of both the window'and the object must becurved. This window shape is defined by the equation:

wherein b is the radius of the wire ring d.

a is the distance from the center of the object to the center of thescreen.

T=tan 29 x and y are the coordinates on the x and y axes, respectively.

Inasmuch as the above equation is involved and ditiicult to handle, itwill be found more convenient to draw the curve of the windowmechanically with the aid of the apparatus illustrated in Fig. 2 whereinthe letters a, b, and hlt have the same meaning as in Fig. l.

The apparatus of Fig. 2 consists of two flat pieces of metal, each bentat R and R to the angle 20. The pivots at R and R' are xed. At the endsof arm 1 are the slots S181 and S1S1". At the ends of arm 2 are theslots S252 and S2"Sz". A line hh' is drawn parallel to the line RR' atdistance a. The distance between the stationary pivots R and R is 2b. Apointer tits snugly into the slots SiSi, S282 at position e. As thepointer e is forced to travel over the straight line hh the pencil at Pis forced by the slots S1"S1, S2S2" to trace a curve designating theshape of the window.

The magnification of my microscope M is given by the equation:

above. Thus, while the magnification is not a constant, the variation ofthe magnication dl dy is zero at the'origin and increases gradually in anegative sense. This means that the magnilication is practicallyconstant near the center and decreases to zero as one approaches y=b. Itis desirable therefore, that the size of the object and window be smallin relation to b.

lThe angle 2H is the angle of strongest diffraction from the materialchosen for the wire. The design of the circular ditracting wire ischiefly dependent on three variables: the spacing of the difractingcrystal planes, the wave length of the X-radiation used, and the desiredangle of divergence of the ditfracted X-rays from the surface of thewire.- If copper Ka radiation (CuKa=l.54l8

A.) is used and brass is the diifracting material with the 2.08 A.spaced planes being the diffracting planes, the Bragg angle (ascalculated from the equation: n `=2d sin 0) will be about 43.5. Theradius of the circular wire is related to this angle 26 the distance aand the size of the window T.

Thus I have demonstrated that a circular wire may be used forenlargement of objects using X-radiation. The variation of opacity ofthe object over its area determines the variation of density over thearea of the photograph and the accuracy of the instrument is nowdetermined by the accuracy to which the wire can be formed into aperfect circle and the neness of the wire and the size and shape of thewindow.

It will be apparent to one skilled in the art that my apparatus formaking enlarged images could be improved by a combination and expansionof the principles developed above. When the area of the target window issmall, additional concentric wires may be added, each made of a dierentmaterial and possessed with its own characteristic (strongestdiffraction) angle 20. This variation is shown in Figure lb.Unfortunately, there is no general equation that will dene a series ofconcentric wires all of which will cooperate to focus the image on theplate. The shape of each additional wire must be lll 4 calculatedseparately and corrected for aberration by the method used in optics todetermine refraction zones.

The method and apparatus of the present invention should not be thoughtof as producing enormous magnications. This is not the purpose of theinvention, and

the precision of surface and thickness of diifracting wiresl set a limitas does the length of exposure. In general, the invention is much moreuseful for magnications of about diameters than it is for greaterdegrees of magnification.

The duration of exposure of the X-ray film is a matter that requiresconsideration. While supercially the overall output of the device andmethod of the present invention may look somewhat the same as thatobtained with refracting optics and ordinary light, the energyefficiency with X-rays is very much lower. X-ray diffraction is arelatively inefficient process and only a small portion of thediffracted rays from any point in the diifracting layer is utilized. Theexposure may be decreased, however, by increasing the number ofconcentric Wires, up to a certain limit. Y

A most important factor is the nature of a diffracting surface. Sinceconvergence results from a diffraction phenomenon and not from specularreflection, the X-rays penetrate through the surface of the wire ringand are diifracted at dierent levels inthe surface to produce parallelbeams. There will be a blurring or lack of definition due to thebroadening eiect of a series of parallel beams diiiracted at differentlevels in the surface. This lack of definition may be visualized as abroadening of sharp lines or the formation of halos about points in theenlarged image. which is determined by the magnification to be used andhence by the resolution required. The thickness ofthe diiracting layer,therefore, should be of the same order of magnitude as the resolution,that is, a 50-diameter magnification would require a layer of the orderof magnitude of five microns. v

Another solution to this problem is to use a wire of microcrystallinemetal of very high absorption, such as lead. Here, the beams diiractedfrom lower levels suffer so much energy absorption that the halosresulting are too dim to interfere seriously with the resolution of animage on a suitable lm. Lead is not a convenient structural materialbecause of its lack of rigidity, but lead supported by more rigidmaterials, such as steel, glass and the like, may be used.

The loss in sharpness resulting from diffraction within a relativelythick diifracting layer on the rings, discussed above, would indicatethe desirability of using very soft X-rays which penetrate very poorly.The choice of an aluminum target will produce X-rays having a wavelength of about l0 A. However, when the radiation is too soft,

other complications are encountered as it is difficult to obtainpowerful diffraction in sharp lines. Thus, for example, X-radiation of awave length of 10 A. might require diifracting substances having aninterplanar spacing of 16 A., which is found mainly in crystallineorganic compounds. I prefer to use metallic diffractors. Radiation of l0A. is a desirable wave length, although vthe invention is stilloperative with even softer X-rays.V As a resultof the diicultiesencountered when too soft or too hard X-radiation is used, the practicaloperating range is X-rays of l to l0 A. Where the nature of the specimenpermits, the best overall combination of resolution and intensity is amonochromatic radiation of wave length between 3 and 5 A.

lt should be noted that the wave length of the X-radiation is notindependent of the nature of the specimen to be examined because certainmaterials have very high absorption characteristics for particular wavelengths due to liuorescence phenomena. For example, copper radiation(Ka=l.54l8 A.) is not suitable for theY examination of iron samplesbecause of excessive scattering of new radiation. It is thereforenecessary that the X-ray source This problem requires a compromise emitrays which have a suitable wave length for the material to beinvestigated, and this will also aect the material and diameter of thewire ring. In order to obtain strong diifraction of the properradiation, these three factors must be taken into consideration. It isof course possible to use a comparatively few diffraction systems tocover the most frequently used X-ray sources and these will permitexamination of a wide variety of samples. The quality of the image maybe improved by introducing a shielding system to cut out undesirablediiraction that occurs at angles other than 20.

I claim:

l. As a new article of manufacture useful for the production of enlargedimages by means of X-ray diffraction, in combination, an X-ray tubecharacterized by a target which functions both as a source of X-rays andas a window for X-ray transmission; planar concentric discs arranged toform at least one annular ring between the circumference of the internaldisc and the inner edge of the external discs; said concentric discsbeing so positioned in a plane parallel to the surface of said targetthat X-rays emerging perpendicularly to t'ne surface of said target atits center pass through the center of said concentric discs; a circle ofwire positioned symmetrically in the annular ring between saidconcentric discs; said circle being of such radius that when an objectis placed adjacent to the external surface of said target an enlargedimage of said object is formed.

2. As a new article of manufacture useful for the production of enlargedimages by means of X-ray diffraction, in combination, an X-ray tubecharacterized by a target which functions both as a source of X-rays andas a window for X-ray transmission; a planar concentric systemcomprising an internal disc surrounded by an annular wire and anexternal disc; and the external disc having a concentric circularopening of a diameter greater than the annular wire; said planarconcentric system being so positioned in a plane parallel to the surfaceof said target that X-rays emerging perpendicularly to the surface ofsaid target at its center pass through the center of said concentricsystem; said annular wire being of such diameter that when an object isplaced on the external surface of said target an enlarged image of saidobject is formed.

3. A new article of manufacture according to claim 2 having more thanone but less than six alternating annular wires and discs in saidconcentric system.

4. As a new article of manufacture useful for the production of enlargedimages by means of X-ray diffraction, in combination, an X-ray tubecharacterized by a target which functions both as a source of X-rays andas a window for X-ray transmission; planar concentric rings arranged toferm au annular space between the circumference of the internal ring andthe inner edge of the external ring, said concentric rings being sopositioned in a plane parallel to the surface of said target that thesource of X-rays, center of said target, and center of said planarconcentric rings are co-linear.

References Cited inthe le of this patent UNITED STATES PATENTS 1,622,149St. `lohn Mar. 22, 1927 1,967,869 Coolidge July 24, 1934 1,993,058 HahnMar. 5, 1935 2,168,780 Olshevsky Aug. 8, 1939 2,440,640 Marton Apr. 27,1948 2,500,948 Kaiser Mar. 21, 1950 2,617,942 McLachlan, Jr Nov. 11,1952 2,653,249 Harker Sept. 22, 1953 2,679,474 Pajes Mar. 25, 1954

1. AS A NEW ARTICLE OF MANUFACTURE USEFUL FOR THE PRODUCTION OF ENLARGEDIMAGES BY MEANS OF X-RAY DIFFRACTION, IN COMBINATION, AN X-RAY TUBECHARACTERIZED BY A TARGET WHICH FUNCITONS BOTH AS A SOURCE OF X-RAYS ANDAS A WINDOW FOR X-RAY TRANSMISSION; PLANAR CONCENTRIC DISCS ARRANGED TOFORM AT LEAST ONE ANNULAR RING BETWEEN THE CIRCUMFERENCE OF THE INTERNALDISC AND THE INNER EDGE OF THE EXTERNAL DISCS; SAID CONCENTRIC DISCSBEING SO POSITIONED IN A PLANE PARALLEL TO THE SURFACE OF SAID TARGETTHAT X-RAYS EMERGING PERPENDICULARLY TO THE SURFACE OF SAID TARGET ATITS CENTER PASS THROUGH THE CENTER OF SAID CONCENTRIC DISCS; A CIRCLE OFWIRE POSITIONED SYMMETRICALLY IN THE ANNULAR RING BETWEEN SAIDCONCENTRIC DISCS; SAID CIRCLE BEING OF SUCH RADIUS THAT WHEN AN OBJECTIS PLACED ADJACENT TO THE EXTERNAL SURFACE OF SAID TARGET AN ENLARGEDIMAGE OF SAID OBJECT IS FORMED.